The invention relates to an optical element comprising a substrate which is provided with a light-transmitting layer the transmission of which in the visible range varies in response to a variation in light.
The invention also relates to a display device comprising a display window which is provided with said optical element.
Optical elements for varying the transmission of light are used to influence the transmission and/or reflection of (visible) light, for example, of lamps, of rear view mirrors and of sunroofs for cars, or of windows for buildings ("smart windows"), or of lenses, or of spectacle lenses for (sun) glasses. They are also used on the viewing side of display windows of (flat-panel) display devices, such as cathode ray tubes (CRTs), plasma display panels (PDPs) and liquid-display devices (LCDs, LC-TVs and plasma-addressed LCDs) to improve the contrast of the image reproduced. By virtue thereof, the feasibility of readily bringing the light transmission to a desired value is increased and, for example, the necessity of changing the glass composition of the display window of a display device is avoided.
The light-transmitting layers mentioned in the opening paragraph influence the intensity of reflected ambient light and the intensity of light originating from an (internal) light source, such as the phosphors in a CRT. Incident ambient light passes through the layer and is reflected, for example, at the substrate or at the phosphors of color filters in a CRT, whereafter the reflected light passes through the layer again. If the transmission of the layer amounts to T, the intensity of the reflected ambient light decreases by a factor of T.sup.2. Light originating from the internal light source(s), however, passes through the light-transmitting layer only once, so that the intensity of this light decreases only by a factor of T. The combination of these effects leads to an increase of the contrast by a factor of T.sup.-1.
Examples of optical elements for varying the transmission of light include, inter alia, electrochromic elements and photochromic elements.
Oxides of specific transition metals are capable of accepting guest atoms such as hydrogen and alkali-metal atoms. If the oxide forms part of an electrochemical cell, the guest atoms can be accepted and released again in a reversible manner. In general, an electrochromic element comprises a first (transparent, conductive) electrode which is connected to an electrochromic layer, the so-called work electrode, and a second (transparent, conductive) electrode, the so-called counter electrode, which contains a material which serves as a source and as an acceptor for the guest atoms, and an ion-conducting (liquid, polymeric or solid) material, the so-called electrolyte, being present between said two electrodes. The transmission properties of the electrochromic element in the visible range undergo a change when a voltage difference is applied across the electrochromic element. As the variation in light is often detected via a light sensor provided in the vicinity of the electrochemical cell, this is referred to as an indirect response to a variation in light.
A photochromic element comprises a layer which contains a material whose transmission varies (automatically) as a result of electromagnetic radiation, such as light, which is incident on the layer (direct response to a variation in light). A large number of photochromic materials, which can be placed in various categories (for example spiro-pyranes, spiro-oxazines or fulgides) are known from the relevant literature. Such a photochromic element enables, for example, the contrast of a (luminescent) image to be increased by providing a photochromic layer on the viewing side of the display window of a display device, the local transmission of the layer in the visible range being governed by the radiation which is (locally) incident on the layer. The transmission of the photochromic element depends (preferably) on radiation which impinges on the layer and the wavelength of which is outside the range in which the display device emits light (for example: the display device emits light in the visible range, while the photochromic element is sensitive to light from the so-called UV-A range) and said transmission decreases automatically as the intensity of the incident light increases.
An optical element of the type mentioned in the opening paragraph is disclosed in U.S. Pat. No. 5,060,075 (PHA 40,577), in which the contrast of a luminescent image is increased by providing a front panel which is arranged in front of a display window of a CRT display device with a light-transmitting layer (for example an electrochromic element), which reduces the reflection of ambient light if the brightness of said ambient light increases. A light sensor, which is provided in the vicinity of the display window, detects variations in ambient light, and a control circuit, which is electrically connected to the light sensor and the front panel, produces, while the display device is in operation, such a control signal that the degree to which the front panel passes light decreases as a function of the increase of the illumination or the intensity of the ambient light (indirect response to a variation in ambient light).
Such optical elements often have the disadvantage that the contrast of the image reproduced is not optimal.